In the contactless drying of a web-formed material, for example pulp, the web-formed material is moved back and forth through a plurality of drying decks with intermediate turning rolls. The drying decks comprise lower blow boxes which at their upper sides blow out process air and usually also upper blow boxes which at their lower sides blow out process air. Usually, the lower blow boxes are designed in such a way that they provide a fixed, stable position for the web-formed material above the lower blow boxes whereas the blow-out from the upper blow boxes occurs perpendicular to the web. The process air from the lower blow boxes thus have a twofold purpose. In addition to drying the web, a stable web run is to be achieved. The only task of the process air from the upper blow boxes is to dry the web-formed material.
In the control of the drying, there are essentially three parameters. The moisture content, the temperature and the volume flow of the process air may be influenced.
The water which escapes from the web-formed material in the form of steam is mixed with and discharged by the process air. To be able to maintain the drying power, part of the process air must therefore be discharged as exhaust air and be replaced by drier, and preferably hot, supply air. This normally occurs to such a limited extent that such a high moisture content in the exhaust air is maintained that condensation and corrosion on exposed parts may only just be avoided. The main part of the process air is recirculated. The volume of exhaust air, corresponding to the discharged volume of process air, other air introduced and any leaked-in air, is adapted such that the moisture content in the exhaust air is controlled against a set value, which is as high as possible in view of the risk of condensation etc. The temperature of the exhaust air may, for example, be 100-130° C. and the water content thereof 0.15-0.30 kg/kg of dry air, and the corresponding temperature and water content of the supply air may, for example, be 75-105° C. and 0.005-0.03 kg/kg, respectively.
The process air is heated by supplying heat to the mixture of supply air and recirculated process air. This normally takes place by recuperative heat exchange wherein the heating medium is low-pressure steam or medium-pressure steam. In the case of an increased drying requirement, the supply of heat is increased and in the case of a reduced drying requirement, the supply of heat is decreased. The temperature of the process air is influenced in an upward direction by an increased supply of heat and in a downward direction by a reduced supply of heat. In the following this is described such that the temperature is controlled although this does not entail a direct control of the temperature which is influenced, inter alia, by the water content in the process air and the degree of recirculation.
Within the framework provided by a maximum moisture content in the exhaust air and a possible supply of heat in the recirculated process air, the aim is to use as small a volume flow as possible for the process air because the fans are driven by electric motors and electrical energy is much more expensive than thermal energy. In a pulp mill, the low-pressure steam is often available at practically no cost. The control is relatively slow and insensitive to variations in material quality.
When changing the grade of the web-formed material and upon start-up after a web break, the adjustment takes a relatively long time. This is largely due to the thermal inertia in the heating system.